The present invention concerns a product based on rubber, the process for obtaining it, a tire made from the product and a process for reducing the rolling resistance of the tire.
One of the main concerns of tire manufacturers is to increase the life of tires. In particular, it is important to improve the endurance of tires in relation to oxidizing processes of the rubber compositions, the metallic or textile reinforcement thereof and the interfaces between such compositions and reinforcements in the tires.
A known method of reducing these oxidation phenomena involves restricting the amount of oxygen coming from the tire inflation air or from the atmosphere outside the tire that gets to a zone of the tire cover that is particularly sensitive to oxidation. For a very long time this has been accomplished using a layer of butyl rubber, which is substantially impermeable to oxygen, that is applied to the inside wall of the tire covers. Unfortunately, butyl rubber is not totally impermeable and the flow of oxygen into the body of the cover, though reduced, produces undesirable oxidation phenomena over the long term.
Other materials that are even more impermeable than butyl rubber have been proposed for the same purpose, as described in, for example, U.S. Pat. Nos. 5,236,030 4,874,670, 5,036,113, 5,040,583 and 5,156,921 and European Patent Application EP-A 337 279. The materials disclosed in these references, however, are expensive and their use in tire covers is associated with a number of problems.
Another way to avoid oxidation problems is to trap the oxygen chemically by the accelerated thermal oxidation of a rubber composition that acts as a buffer, located between a main source of oxygen and the zone to be protected against oxidation phenomena. For example, such a buffer composition may be located between the inside face of the tire cover, coated with butyl rubber, and the carcass ply to reduce the amount of oxygen that comes into contact with the carcass ply from the inflation air, especially in tire covers intended for fitting to vehicles that carry heavy loads.
To accelerate oxygen fixation, such buffer compositions may contain a metallic salt that catalyzes oxidation, in particular a cobalt salt. The salt acts by activating homolytic decomposition of the hydroperoxides generated during aging brought about by the aforementioned said oxidation phenomena. The salt is introduced into the buffer composition, preferably in amounts of 0.2 to 0.3 pbw (parts by weight of cobalt equivalent per 100 parts by weight of the elastomer). This increases the amount of oxygen that can be trapped by the buffer composition by around 50 to 100% compared with the same composition containing no cobalt salt.
Unfortunately, improvement in the oxidation-related behavior is accompanied by a substantial increase of the hysteresis losses of the buffer composition because of the considerable quantity of cobalt salt introduced. This increase of the hysteresis losses leads to self-heating of the composition, which results in a shorter life, contrary to the purpose intended, and an undesirable increase of the rolling resistance, which should also be avoided since decreased rolling resistance provides reduced fuel consumption.
For these reasons the use of such buffer compositions, attractive while potentially, has not developed as expected.
European patent application EP-A-507 207 describes a method for trapping oxygen by means of an elastomeric buffer composition contained in a wrapping layer. The buffer composition is characterized by the presence of a transitional metal salt provided to activate oxygen fixation. As explained above, cobalt salts are the preferred metal salts, with, other metals, such as manganese or iron also envisaged, but not in relation to specified salts.
The present invention provides a rubber-based product useful as a tire cover which comprises one or more buffer zones being provided in order to trap oxygen external to the product and one or more zones of the tire cover that are sensitive to oxidation phenomena (sensitive zones), wherein the buffer zone protects the sensitive zones of the product from oxidation. In accordance with the invention, the buffer zone of the product contains a basic composition composed of at least one elastomer containing at least one salt of iron (III) provided to activate oxidation in the composition. Preferably the salt is an iron (III) salt of a carboxylic acid having the general formula CnH2nO2 in which n is 2 to 5. Particularly preferred salts include iron (III) pentanoate and iron (III) acetate.
In accordance with the invention, the quantity of the invention iron (III) salt in the composition preferably ranges from 0.01 to 0.03 pbw of equivalent iron, where xe2x80x9cpbwxe2x80x9d means xe2x80x9cparts by weight per 100 parts by weight of the elastomer or totality of elastomers present in the compositionxe2x80x9d. More preferably still, the quantity of iron (III) salt invention ranges from 0.01 to 0.02 pbw of equivalent iron.
The buffer zone composition is prepared incorporating the iron (III) salt into the elastomer or elastomers by mechanical working.
The buffer zone composition is advantageously applied in a tire cover and inparts improved rolling resistance to the tire.
The buffer zone composition according to the invention is based on natural or synthetic rubber, or a blend of two or more such rubbers. Synthetic rubbers suitable for use in the composition according to the invention include diene rubbers, such as polyisoprene and polybutadiene, mono-olefin rubbers, such as polychloroprene and polyisobutylene, styrene-butadiene and styrene-butadiene-isoprene copolymers, acrylonitrile-butadiene-styrene, copolymers and ethylene-propylene-diene terpolymers. Diene rubbers are preferred synthetic rubbers, in particular any homopolymer obtained by polymerization of a conjugated diene monomer having 4 to 12 carbon atoms, or any copolymer obtained by co-polymerization of one or more dienes conjugated either between themselves or with one or more vinyl aromatic compounds having from 8 to 20 carbon atoms.
Suitable conjugated dienes include, 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C1 to C5 alkyl)-1,3-butadienes such as, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene and phenyl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene.
Suitable vinyl aromatic compounds include, styrene, ortho-, meta- and para-methylstyrene, the commercial mixture xe2x80x9cvinyl-toluenexe2x80x9d, para-tertiobutylstyrene, the methoxy-styrenes, the chloro-styrenes, vinylmesitylene, divinyl benzene, vinyl naphthalene, etc.
The co-polymers may contain between 99% and 20% by weight of diene units and between 1% and 80% by weight of vinyl aromatic units.
The polymers may have any microstructure, which is a function of the polymerization conditions used, in particular the presence or absence of a modifying and/or randomizing agent and the quantities of modifying and/or randomizing agent used. The polymers include block, statistical, sequenced or micro-sequenced polymers, and may be prepared in a dispersion or in solution.
Preferred synthetic diene rubbers include polybutadienes, in particular those having a content of 1,2-units between 4% and 80% and those having a content of cis-1,4 bonds of more than 90%, the polyisoprenes, and butadiene-styrene co-polymers, in particular those having a styrene content between 5% and 50% by weight, more particularly between 20% and 40% by weight, a content of 1,2-bonds of the butadiene part between 4% and 65%, and a content of trans-1,4 bonds between 30% and 80%, these having a total aromatic compound content between 5% and 50%, and a glass transition temperature (Tg) between 0xc2x0 C. and 80xc2x0 C., and those having a styrene content of between 25% and 30% by weight, a content of vinyl bonds of the butadiene part between 55% and 65%, a content of trans-1,4 bonds between 20% and 25% and a glass transition temperature between xe2x88x9220xc2x0 C. and xe2x88x9230xc2x0 C.
Suitable butadiene-styrene-isoprene co-polymers, include those having a styrene content between 5% and 50% by weight, more particularly between 10% and 40%, an isoprene content between 15% and 60% by weight, more particularly between 20% and 50% by weight, a butadiene content between 5% and 50%, more particularly between 20% and 40% by weight, a content of 1,2-units of the butadiene part between 4% and 85%, a content of trans-1,4 units of the butadiene part between 6% and 80%, a content of 1,2-plus 3,4-units of the isoprene part between 5% and 70%, and a content of trans-1,4 units of the isoprene part between 10% and 50%.
The synthetic rubbers may be coupled and/or starred or alternatively functionalized with a coupling and/or starring or functionalizing agent.
These rubbers may be vulcanized and/or cross-linked by any of the known agents, including as sulphur, peroxides, and bismaleimides.
The composition according to the invention contains usual fillers and additives, such as carbon black, silica or any other reinforcing filler, stearic acid, reinforcing resins, zinc oxide, activators, pigments, vulcanization accelerators or retarders, anti-aging agents, such as anti-oxidants, anti-reversion agents, oils or various agents to facilitate use, tackiness promoting resins, metal adhesion promoters, anti-ozone waxes, silicon binding and/or covering agents, etc.
The compositions according to the invention can be used in a wide variety of applications, notably for numerous rubber products. In particular the compositions are used in tire covers as buffer compositions between a source of oxygen, in particular inflation air or the external atmosphere and a zone to be protected in the tire cover. For example, these compositions may be used inside the internal calendering rubber, between the calendering rubber and the carcass ply, between the carcass ply and the crown plies, between the crown plies and the tread, between the carcass ply and the side walls, or even on the outside of the side walls.
A tire cover according to the invention, wherein the tire cover comprises internal calendering rubber, a carcass ply extending from one bead wire to the other, crown plies, side walls ending in beads comprising at least one bead wire, and a tread, is characterized such that a buffer zone containing the composition of the invention occupies at least one of the following positions: radially inside the said calendering rubber, between the internal calendering and the carcass ply, between the carcass ply and the crown plies, between the crown plies and the tread, between the carcass ply and the side walls, inside or outside the side walls, and inside or outside the tread.
The use of the iron compound according to the invention is very different from the known uses of iron compounds in the rubber industry, such as their use as oxidizing salts to promote the mastication of rubbers (peptizing properties) or devulcanization for recycling, as described, for example, U.S. Pat. No. 3,324,100, European patent application EP 157 079 A and Russian application RU 2,014,339 A.
The process for obtaining a rubber-based product according to the present invention, comprises incorporating the iron (III) salt in the elastomer or elastomers of the buffer composition by working the salt into the elastomer(s) in mechanically to obtain the corresponding buffer zone.
Preferably the iron (III) salt is incorporated into in the said elastomer(s) at the same time as a reinforcing filler is incorporated.
The process for reducing the rolling resistance of a tire cover, comprises incorporating an iron (III) salt as defined above in an elastomer or in the elastomers constituting the tire cover, by working the salt in mechanically.
The invention will be easily understood with the help of the non-limiting examples given below.
These examples are either examples according to the invention, or ones not according to the invention that use compositions without any metallic derivative, or those containing cobalt salts iron (III) salts, other than an iron (III) salt of a carboxylic acid having the formula Cn.H2n O2 in which n is from 2 to 5.
The oxidation-promoting efficacy of the iron or cobalt compounds is assessed by subjecting the compositions to aging by thermal oxidation. The oxygen uptake is then measured by elemental analysis and the changes in mechanical properties, such as the modulus, hysteresis loss and rupture properties, are determined.
Vulcanization
Unless otherwise indicated, all the tests were carried out on compositions that had been vulcanized by curing for 20 to 30 min at 150xc2x0 C.
Aging by Thermal Oxidation
A ventilated stove at 85xc2x0 C. is used. This temperature is regarded as representative of the temperatures encountered during the operation of tire covers.
Hysteresis Loss
The hysteresis loss, or hysteresis (Ph) is measured by determining the energy lost on rebound compared with the input energy at 60xc2x0 C., measured at the sixth shock. The value, expressed as a percentage, is the difference between the energy supplied and the energy returned, referred to the energy supplied. The deformation for the losses measured is 40%.
Tensile Tests
The strain modulus at 10% strain (M10) and at 100% strain (M100) are determined, in accordance with the standard ISO 37.
Scott Fracture Indexes
The rupture stress (Cr) in MPa and the elongation at rupture (Ar) in % are also determined. All these tensile measurements are carried out under normal temperature and humidity conditions according to ISO 37.